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Brain evolution and other Mesozoic mammal stuff



From: Ben Creisler
bh480@scn.org


Some recent items about Mesozoic mammals.

In the new issue of Science:

R. Glenn Northcutt (2011)
Perspective Paleontology: Evolving Large and Complex Brains.
Science 332 (6032): 926-927 (20 May 2011): 
DOI: 10.1126/science.1206915 
http://www.sciencemag.org/content/332/6032/926.summary

Summary
During the Mesozoic (∼250 million to 65 million years ago), two
distantly related groups of reptiles?the cynodont (or mammal-like) reptiles
and the coelurosaurian theropod dinosaurs?gave rise to mammals and birds,
respectively. Both mammals and birds evolved brains some 10 times as large,
relative to a given body weight, as those of their ancestors (1). In both
groups, these brains contributed to the evolution of the ability to control
body temperature (endothermy) and complex social interactions, including
parental care and a reliance on learning that even involves tool use (2,
3). The size of most parts of the brain increased in birds and mammals, but
the cerebral hemispheres and cerebellum, both of which are involved in
sensory and motor integration, underwent particularly spectacular
development (see the figure). Although mammals and birds evolved from
distantly related groups of reptiles, the higher integrative centers and
circuitry of their cerebral hemispheres are very similar, and comparative
neurobiologists continue to vigorously debate whether these centers evolved
from the same ancestral neural centers (4, 5) or from different ones (6?8).
Speculation about the evolutionary steps leading to large and complex
mammalian and avian brains is equally contentious and unresolved, in part
because of the rarity of fossil skulls and, until recently, the need to
destroy such skulls in order to expose the endocasts (casts molded by the
cranial cavity). Typically, endocasts are the only record of the brain's
outward appearance in a transitional form, because brains themselves are
rarely fossilized. 

Timothy B. Rowe, Thomas E. Macrini, and Zhe-Xi Luo (2011) 
Fossil Evidence on Origin of the Mammalian Brain.
Science 332(6032): 955-957 (20 May 2011): 
DOI: 10.1126/science.1203117 
http://www.sciencemag.org/content/332/6032/955.abstract

Abstract
Many hypotheses have been postulated regarding the early evolution of the
mammalian brain. Here, x-ray tomography of the Early Jurassic mammaliaforms
Morganucodon and Hadrocodium sheds light on this history. We found that
relative brain size expanded to mammalian levels, with enlarged olfactory
bulbs, neocortex, olfactory (pyriform) cortex, and cerebellum, in two
evolutionary pulses. The initial pulse was probably driven by increased
resolution in olfaction and improvements in tactile sensitivity (from body
hair) and neuromuscular coordination. A second pulse of olfactory
enhancement then enlarged the brain to mammalian levels. The origin of
crown Mammalia saw a third pulse of olfactory enhancement, with ossified
ethmoid turbinals supporting an expansive olfactory epithelium in the nasal
cavity, allowing full expression of a huge odorant receptor genome. 


J. David Archibald, Yue Zhang, Tony Harper and Richard L. Cifelli (2011)
Protungulatum, Confirmed Cretaceous Occurrence of an Otherwise Paleocene
Eutherian (Placental?) Mammal. 
Journal of Mammalian Evolution (advance online publication)
DOI: 10.1007/s10914-011-9162-1 
http://www.springerlink.com/content/n661405803q511n8/


Neither pre-Cenozoic crown eutherian mammals (placentals) nor archaic
ungulates (?condylarths?) are known with certainty based on the fossil
record. Herein we report a new species of the Paleocene archaic ungulate
(?condylarth?) Protungulatum from undisputed Late Cretaceous aged rocks in
Montana USA based on an isolated last upper premolar, indicating rare
representatives of these common early Tertiary mammals appeared in North
America a minimum of 300 k  years before the extinction of non-avian
dinosaurs. The other 1200 mammal specimens from the locality are
characteristic Late Cretaceous taxa. This discovery overturns the current
hypothesis that archaic ungulates did not appear in North America until
after the Cretaceous/Tertiary (K/T) boundary and also suggests that other
reports of North American Late Cretaceous archaic ungulates may be correct.
Recent studies, including ours, cannot determine whether Protungulatum does
or does not belong to the crown clade Placentalia. 


A few months old but its free--pdf of short survey article about Chinese
Mesozoic mammals:
http://english.ivpp.cas.cn/rh/as/201012/P020101207396650166902.pdf

A. O. Averianov and A. V. Lopatin (2011)
Phylogeny of triconodonts and symmetrodonts and the origin of extant
mammals. 
Doklady Biological Sciences 436(1): 32-35, 
DOI: 10.1134/S0012496611010042 
http://www.springerlink.com/content/3x027744038g6866/
(no abstract)



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